We know that our climate system is changing as global temperatures rise. It is also now possible, in many cases, for science to credibly speak to the influence of climate change on the likelihood and/or the extent/severity of a certain type of event. However, according to a pre-publication version of a new National Academies of Sciences (NAS) report, there is still a long way to go to make credible claims about how much and in what ways a particular extreme weather event was affected by climate change.

The NAS report, Attribution of Extreme Weather Events in the Context of Climate Change (free pdf download available), is the work of the Committee on Extreme Weather Events and Climate Change Attribution. To create it, the Committee drew on more than 300 published papers, and conducted a 3-month process of multiple webinar meetings and a community workshop with leading scientists and other researchers working in the arena of event attribution.

The report provides the most up to date assessment of current capabilities in event attribution, guidance on presenting and interpreting studies, and priorities for both future research and its application (operationalization). While the report notes significant gains in the science of extreme event attribution, especially over the past decade, it cautions that this science is still emerging, and substantially more study is required.

On methodology, the report identifies two classes of approaches to event attribution – observational-based and model-based – noting that most studies use both to varying degrees. The model-based approach must, of course, account for multiple uncertainties, and the report looks at how those have been quantified. The suite of “individual classes of extreme events” examined includes extreme heat and cold events, droughts, wildfires, extreme rainfall, cyclones, and more.

The authors assign a confidence level (high, medium, low) to the attribution science for each of these event classes by evaluating three different measures:

• “the capabilities of climate models to simulate an event class,
• the quality and length of the observational record from a climate perspective, and
• understanding of the physical mechanisms that lead to changes in extremes as a result of climate change.”

The attribution analyses of extreme heat and cold events garner the highest level of confidence. There is medium confidence in those of hydrological drought and heavy precipitation, and little or no confidence in those of severe convective storms and extratropical cyclones. (See Fig. S.4) (Severe convective storms are severe thunderstorms, often characterized by hail, lightning, and/or high wind gusts. Extratropical cyclones are low pressure, generally mid-latitude systems associated with cold or warm fronts, e.g., blizzards, Nor-easters.)

(AP Photo/Tony Gutierrez)

In particular, the authors caution that, “there is no single best method or set of assumptions for event attribution.” How the question is framed and the time constraints imposed on a study play a significant role in the choices made for such parameters as: defining event duration, setting geographical area affected, identifying the physical variables to study, and choosing methodology. Natural variability is also always a player in an extreme event. Instead of “Did climate change cause this event?”, the authors suggest reasonable questions might be: “Are events of this severity becoming more or less likely because of climate change?” and “To what extent was the storm intensified or weakened, or its precipitation increased or decreased, because of climate change?”

The report recommends more study of 9 specific areas of weather and climate extremes, suggests the creation of standards based on event classes, and proposes ways to improve systemic evaluation.

While policymakers and the public need the science in order to better manage the risks around these events and enhance our adaptive capacity, those most vulnerable to climate change are looking toward the day that blame for these events can be apportioned and thus restitution sought. That day is getting closer, but it is definitely not here yet.